Code simplification

The following code https://github.com/carlhuda/cargo/blob/master/src/bin/cargo-read-manifest.rs#L47-L49 can be re-written as:

```
for err in decoder.error.iter() {
    fail!("Error: {}", err);
}
```

or

```
decoder.error.map(|err| fail!("Error: {}", err));
```
